Group 17 Elements: Properties and Reactions

Vocabulary flashcards covering trends in volatility, bond strength, oxidising and reducing power, hydrogen halide stability, halide identification tests, and disproportionation reactions of Group 17 halogens.

Group 17 Elements

The group of non-metal elements in the periodic table also known as the halogens.

Volatility Trend of Group 17

Volatility decreases down the group because the number of electrons and molecular size increase, leading to stronger temporary dipoles, increased van der Waals forces, and a requirement for more energy to overcome these intermolecular attractions.

Halogen Covalent Bond Strength

The strength of covalent bonds between halogen atoms decreases down the group because atomic radius and electron shielding increase, placing the shared pair of electrons further from the nucleus and weakening the electrostatic attraction.

Halogen Oxidising Ability

The ability of halogens to act as oxidising agents by gaining an electron decreases down Group 17 ($Cl_2 > Br_2 > I_2$) due to increased atomic radius and electron shielding, which weakens the attraction between the nucleus and the incoming electron.

Halogen Displacement Reaction

A reaction in which a more reactive halogen displaces a halide ion from solution if the halide ion is located below it in the periodic table (e.g., $Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2$).

Halogen Reaction with Hydrogen

The reaction of halogens ($X_2$) with hydrogen ($H_2$) to form hydrogen halides ($X_2 + H_2 \rightarrow 2HX$), which decreases in reactivity down the group: fluorine reacts explosively in the cold, chlorine reacts with light heating or sunlight, bromine requires a flame, and iodine only partially reacts with constant heating.

Thermal Stability of Hydrogen Halides

A measure of how easily a hydrogen halide is broken down into its constituent elements upon heating; it decreases down Group 17 as the halogen atom size increases, resulting in longer, weaker $H-X$ covalent bonds with lower bond enthalpies.

Halide Ion Reducing Ability

The ability of halide ions to act as reducing agents by losing an electron; it increases down Group 17 ($I^- > Br^- > Cl^- > F^-$) because increased atomic radius and shielding make outer electrons less strongly held by the nucleus.

Silver Nitrate Test for Halide Ions

A qualitative test where dilute nitric acid ($HNO_3$) is first added to remove interfering ions such as carbonate, followed by silver nitrate solution ($AgNO_3$) to precipitate silver halides ($Ag^+_{(aq)} + X^-_{(aq)} \rightarrow AgX_{(s)}$).

Chloride Ion ($Cl^-$) Precipitate Identification

Forms a white precipitate of $AgCl$ upon addition of silver nitrate, which dissolves completely in dilute aqueous ammonia ($NH_3$).

Bromide Ion ($Br^-$) Precipitate Identification

Forms a cream precipitate of $AgBr$ upon addition of silver nitrate, which is insoluble in dilute aqueous ammonia but soluble in concentrated aqueous ammonia ($NH_3$).

Iodide Ion ($I^-$) Precipitate Identification

Forms a yellow precipitate of $AgI$ upon addition of silver nitrate, which is insoluble in both dilute and concentrated aqueous ammonia ($NH_3$).

Reaction of Sodium Chloride with Concentrated Sulfuric Acid

An acid-base reaction producing misty fumes of hydrogen chloride: $NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl$. No redox occurs because $HCl$ is not a sufficiently strong reducing agent.

Reaction of Sodium Bromide with Concentrated Sulfuric Acid

A reaction forming misty fumes of $HBr$ ($NaBr + H_2SO_4 \rightarrow NaHSO_4 + HBr$), followed by a redox reaction where $HBr$ reduces sulfuric acid to produce choking $SO_2$ gas and brown fumes of $Br_2$: $2HBr + H_2SO_4 \rightarrow Br_2 + SO_2 + 2H_2O$.

Reaction of Sodium Iodide with Concentrated Sulfuric Acid

A reaction forming misty fumes of $HI$ ($NaI + H_2SO_4 \rightarrow NaHSO_4 + HI$), followed by redox reduction of sulfuric acid to $SO_2$ and further reduction to toxic $H_2S$ (rotten egg smell) alongside purple/black $I_2$: $6HI + SO_2 \rightarrow H_2S + 3I_2 + 2H_2O$.

Disproportionation Reaction

A redox reaction in which atoms of the same element are simultaneously oxidised and reduced.

Reaction of Chlorine with Cold Dilute Sodium Hydroxide

A disproportionation reaction producing bleach: $2NaOH_{(aq)} + Cl_{2(g)} \rightarrow NaClO_{(aq)} + NaCl_{(aq)} + H_2O_{(l)}$, where chlorine is simultaneously oxidised from $0$ to $+1$ in $NaClO$ and reduced from $0$ to $-1$ in $NaCl$.

Sodium Chlorate(I) ($NaClO$)

The active bleaching agent formed from cold dilute sodium hydroxide and chlorine; widely used in water treatment, textile and paper bleaching, and cleaning products to destroy bacteria.

Reaction of Chlorine with Hot Concentrated Sodium Hydroxide

A disproportionation reaction given by $6NaOH_{(aq)} + 3Cl_{2(g)} \rightarrow 5NaCl_{(aq)} + NaClO_{3(aq)} + 3H_2O_{(l)}$, where chlorine is oxidised from $0$ to $+5$ in $NaClO_3$ and reduced from $0$ to $-1$ in $NaCl$.

Chlorine Water Treatment

The disproportionation reaction of chlorine with water ($Cl_2 + H_2O \rightleftharpoons 2H^+ + Cl^- + ClO^-$) producing chlorate ions that kill bacteria and microorganisms, stop algae growth, remove discolouration and odor, and offer residual antimicrobial protection.